When detecting X-rays by using an X-ray fluoroscopic apparatus, X-rays (to be referred to as scattered rays hereinafter) scattered by an object sometimes enter the detector. A grid is used to prevent scattered rays from entering the detector. FIG. 14 shows how an FPD (Flat Panel Detector) detects X-rays generated from an X-ray tube. The direct radiation indicated by the one-dot dashed line is an X-ray which is not transmitted through an object P. The scattered rays indicated by the dotted lines are transmitted through the object P and scattered by the object P in the process of being transmitted. The unscattered rays indicated by the solid lines, which are transmitted through the object P, are X-rays which are transmitted through the object P without being scattered.
The grid has a pattern structure in which aluminum foils or the like through which X-rays are transmitted and lead foils or the like which absorb scattered X-rays are alternately arranged. As shown in FIG. 14, when X-ray imaging or X-ray fluoroscopy is executed while the grid is placed on the X-ray incident surface of the detector, the grid removes scattered rays. This makes it difficult for scattered rays to reach the detector. This reduces the dose of scattered rays reaching the detector. However, some scattered rays reach the detector without being removed. The scattered rays having reached the detector are displayed as noise on an image (to be referred to as a medical image hereinafter) generated based on an output from the detector. For this reason, a medical image is processed to reduce scattered ray components on the image (this operation will be referred to as scattered ray correction processing hereinafter).
Conventionally, as one type of scattered ray correction processing, there is available a technique of executing scattered ray correction processing in computation in a frequency space. The Fourier transform of an image (to be referred to as a scattered ray reduced image hereinafter) having undergone a reduction in scattered ray component by scattered ray correction processing is performed based on the Fourier transform of a medical image and the Fourier transform of a scattering function. A scattered ray reduced image is generated by applying an inverse Fourier transform to the Fourier transform of the scattered ray reduced image. With the above method, however, it is not possible to change a scattering function in accordance with the position of each pixel in a medical image. It is therefore not possible to properly reduce scattered ray components in, for example, a medical image containing direct radiation components or a medical image or the like having non-direct radiation components which are transmitted through a partially thin portion (to be referred to as a small body thickness portion hereinafter) of an object P. For example, this raises a problem that scattered ray components are excessively corrected (this operation will be referred to as excessive correction hereinafter).
In order to solve the above problem, there is available a method of generating a scattered ray reduced image by executing a repetitive operation a plurality of times to be described below. In this method, when the repeat count is 1, the first scattered ray components are estimated based on a medical image. In this case, the first scattered ray components are calculated by multiplying the convolution sum of a plurality of pixel values in the medical image and a scattering function by a direct radiation ratio. The direct radiation ratio is the ratio of the dose of direct radiation to the sum of the dose of direct radiation and the dose of scattered rays at each pixel in the medical image. The first scattered ray reduced image is then generated by subtracting the first scattered ray components from the medical image.
When the repeat count is 2, the second scattered ray components are estimated based on the first scattered ray reduced image. In this case, the second scattered ray components are calculated by multiplying the convolution sum of a plurality of pixel values in the first scattered ray reduced image and a scattering function by a direct radiation ratio. The direct radiation ratio is the ratio of the dose of direct radiation to the sum of the dose of direct radiation and the dose of scattered rays at each pixel in the first scattered ray reduced image. The second scattered ray reduced image is then generated by subtracting the first scattered ray components from the second scattered ray components.
When the repeat count is n (n is a natural number equal to or more than 2), the nth scattered ray components are estimated based on the (n−1)th scattered ray reduced image. In this case, the nth scattered ray components are calculated by multiplying the convolution sum of a plurality of pixel values in the (n−1)th scattered ray reduced image and a scattering function by a direct radiation ratio. The direct radiation ratio is the ratio of the dose of direct radiation to the sum of the dose of direct radiation and the dose of scattered rays at each pixel in the (n−1)th scattered ray reduced image. The nth scattered ray reduced image is then generated by subtracting the (n−1)th scattered ray components from the nth scattered ray components. A scattered ray reduced image is generated by repeating the above processing a predetermined number of times.
In general, the larger the repeat count n, the higher the accuracy of scattered ray correction processing. According to the above technique, however, as the value of n increases, the number of times of convolution sum calculation increases. This leads to an increase in calculation amount. That is, the above technique has a problem that it takes much time to execute scattered ray correction processing. For example, in X-ray fluoroscopy, display delay or frame drop occurs, resulting in a difficulty in real-time display.
It is an object to provide a medical image processing apparatus, an X-ray diagnostic apparatus, and an X-ray computed tomography apparatus which can generate scattered ray reduced images without excessive correction, while suppressing a calculation amount, from a medical image having direct radiation components and even from a medical image having non-direct radiation components transmitted through a partially thin portion of an object.